The invention relates to an X-ray tube which includes an anode and a cathode arrangement that includes a cathode cup for electron focusing, a flat electron emitter that is provided with openings, and an electrode that is arranged on the side of the electron emitter that is remote from the anode. An X-ray tube of this kind is known from U.S. Pat. No. 4,344,011. The electron emitter in one of the embodiments disclosed therein consists of a plane, flat and meandering metal band. Openings are thus present between the segments to and fro of said metal band.
The potential of the cathode cup in the known X-ray tube is variable relative to the electron emitter, so that faults in the manufacturing process do not have an effect on the dimensions of the focal spot. When the potential on the cathode cup is more positive than that on the electron emitter by a given amount, electrons from the lateral regions or from the back of the electron emitter can reach the cathode cup and heat the latter. Therefore, in one embodiment an electrode is arranged at a small distance from the electron emitter; this electrode shields the back and the lateral regions of the electron emitter and its potential corresponds at least approximately to the potential of the electron emitter.
The advantage of such a flat electron emitter over an electron emitter consisting of a helically wound wire is that the electron trajectories can be focused better, so that a focal spot having a more attractive electron density distribution is formed on the anode. However, the electron density distribution that can be achieved in the focal spot does not reach that of an ideal flat emitter. An ideal flat emitter is to be understood to mean a plane emitter having a thickness zero and no openings.
It is an object of the present invention to provide an X-ray tube of the kind set forth such that the characteristic of the ideal flat emitter is obtained at least approximately.
This object is achieved in accordance with the invention in that the electrode is arranged to be connected to a negative potential relative to the electron emitter in the operating condition of the X-ray tube, which negative potential is so high that the electric field strength in the space between the electrode and the electron emitter amounts to at least 20%, but preferably at least 100%, of the field strength on the side of the electron emitter that faces the anode.
The invention is based on the recognition of the fact that the electric field in the known X-ray tube, extends as far as into the openings, so that the equipotential lines on the surface of the electron emitter that faces the anode are drawn into the openings. Therefore, at the area of the openings there are formed electron trajectories that deviate from those of an ideal flat emitter and make it impossible to achieve the characteristic of such an ideal flat emitter. Because the electrode on the back side of the electron emitter that is remote from the anode carries a negative potential, the equipotential lines are pushed out of the openings again. A suitable choice of the potential enables the equipotential surfaces on the front side of the electron emitter that faces the anode to be substantially plane. The electron trajectories then extend rectilinearly and perpendicularly to the surface of the electron emitter throughout the vicinity thereof.
The above steps increase the ratio of the surface area of the electron emitter to the surface area of the focal spot. A given focal spot size can thus be obtained by means of a larger electron emitter. For achieving a given electron density in the focal spot the electron emitter can be kept at a lower temperature, so that its service life is prolonged. A further advantage offered by the invention is that it is now easier to control the position and/or size of the focal spot.
Preferred embodiments of the invention are set forth in the claims, wherein it is readily apparent that the electron emitter may also have a shape other than that of a meander (for example, the shape of a spiral), but a meander is easier to produce. In another embodiment, the electrode that is situated on the rear of the electron emitter has a better grip on the front of the electron emitter. The electric voltage between the electron emitter and the electrode can thus be reduced for the same distance between these elements.
In another embodiment, a different shape is also possible in principle, for example, a curved shape of the electron emitter. In that case the electrode should be adapted to said curvature.
When the electron emitter occupies a specific position as claimed, the electron emitter and the cathode cup can carry the same potential.
Another embodiment is claimed as an X-ray device which includes an X-ray tube as claimed. Another embodiment disclosed ensures that the bias voltage of the electrode is varied in dependence on the tube voltage (that is, the voltage between the anode and the cathode) in such a manner that the optimum field configuration is always achieved at the area of the electron emitter.